Xerographic apparatus and method



Jan. 17, 1961 Filed March 5, 1958 R. W. GUNDLACH XEROGRAPHIC APPARATUS AND METHOD 3 Sheets-Sheet 1 HIGH VOLTAGE SOURCE FIG. 2

INVENTOR. Robert W. Gundlach ATTORNEY Jan. 17, 1961 R. w. GUNDLACH XEROGRAPHIC APPARATUS AND METHOD 3 Sheets-Sheet 2 Filed March a, 1958 FIG. 5

INVENTOR. Robert W. Gundlach ATTORNEY Jan. 17, 1961 R. w. GUNDLACH 2,958,553

XEROGRAPHIC APPARATUS AND METHOD Filed March 3, 1958 3 Sheets-$heet 3 TO TRANSF ER EL ECT RODE TO CO RO NA 'O'O'O'O LECTRODE TO CO RO NA E LECT RODEH) INVENTOR. Robert W. Gundlach ATTORNEY XERUGRAPHIC APPARATUS AND METHQD Rebert W. Gundlach, Spencerport, N.Y., assignor to Haloid Xerox Inc., a corporation of New York Filed Mar. 3, 1958, Ser. No. 718,693

7 Claims. (Cl. 96-1) This invention relates in general to the formation of developed electric images and in particular to the formation of substantially instantaneous developed electrostatic images as for example in response to the action of light.

This application is a continuation-in-part of my copending application Ser. No. 613,333. In this aforementioned application there is described and claimed methods and apparatus for the formation of immediately visible developed electric images in response to the action of light, where the images are of the same photographic sense as the original pattern of light and shadow. Thus, in the prior application there is described a method by which photographic positive-to-positive copy consisting of dark or black images on a white background can be produced from optical originals consisting of dark images on a light background. These images are produced according to the prior invention by applying a uniform electric charge to a layer of releasably attracted finely divided material on a photoconductor, and selectively transferring image portions of the releasable particle layer to a printreceiving member by the action of light or radiation on the photoconductor combined with the electric attraction of the electric charges on the particles themselves Prior efforts to control photographic contrast and to explore the feasibility of photographic reversal indicated that reversal techniques Were futile. It had been found, for example, that the application of electric field of either polarity reduced the quality of the reproduced image and that increasing the potential of the applied field effectively defeated the process entirely.

Now in accordance with the present invention it has been found that the application of an intense field in a direction and of a polarity to cause transfer of developer material in the background areas and of a field intensity in excess of the intensity causing obscuring of the image produces a reversal of the transfer phenomena. Accordingly, the present invention accomplishes the purpose of producing photographic reversal copy by application of a reverse electric field during the transfer operation whereby the developer material is transferred only in the light struck or bound charge areas.

It is, accordingly, an object of the present invention to provide novel apparatus and methods for forming photographic reversal images.

It is another object of the invention to provide a new method for the formation of photographically reversed images by electrical means.

Additional objects will in part be obvious and will in part become apparent in the following specifications and drawings in which:

Figs. 1-4 are diagrammatic flow sheets of the operation of the invention according to one embodiment thereof;

Fig. 5 is a diagrammatic representation of automatically operating apparatus employing a rotatable cylinder;

Fig. 6 is a circuit diagram of a power supply and switching circuit to produce either direct or reversal copy from the apparatus of Fig. 5.

One method of operation of the present invention is Patented Jan. 17, l dl ice illustrated in Fig. 1 wherein are shown a series of steps and procedures according to which a visible image is formed in direct response to an image of light and shadow to be recorded. As illustrated in Figs. 14 a photoresponsive member generally designated 10, may comprise a photoconductive insulating layer 11 disposed on a conductive support generally designated 12 which may, for example, comprise a conductive coating or layer 13 on a transparent backing support 14. Illustratively, the backing support may be any transparent member such as, for example, glass, transparent plastic or film of any desired sort either inherently conductive or having a conductive coating on at least one surface thereof. For example, partially silvered or metalized glass may be employed as the backing support or glass with other conductive coatings. A presently preferred base support is NESA glass which is understood to be glass having a conductive tin oxide coating on its surface, available from Pittsburgh Plate Glass Company, Pittsburgh, Pennsylvania.

The photoconductor 11 may be any suitable photoconductive layer such as, for example, a photoconductive insulator conventional to the xerographic art. Such photoconductive insulators include vitreous or amorphous selenium deposited by vacuum deposition, spraying, or melt-coating methods to form a photoconductive insulating layer on the support surface. In addition, there may be employed other photoconductors such as anthracene, sulfur, or hinder layers of photoactive materials such as certain sulfides, oxides and selenides of Zinc, cadmium, calcium lead and other elements. A suitable material of this short may be a photoconductive insulating layer comprising a photoactive material such as, for example, zinc oxide in a film-forming binder such as, for example, an insulating silicone resin or the like.

As illustrated in Fig. l, a layer 16 of powder or similar transferable finely divided material is deposited on the surface of the photoactive member as, for example, by deposition from a rotating brush 17. This material is deposited in a form such that it adheres loosely to the plate surface by mild electrostatic forces or similar attractive forces. This can be accomplished by deposition of a suitable material as is conventional in the xerographic developing art. Such deposition may be accomplished as illustrated in Fig. l by deposition from a rotating brush or by deposition from a mixture of fine particles with larger size carrier particles as illustrated for xerographic development in Wise Patent 2,618,552. The

eveloper material or powder may, in addition, be depos ited by a dusting or spray deposition method such as employed in Carlson Patent 2,297,691 for the development of xerographic images. Other deposition methods may be employed as desired to deposit on the plate surface a thin film or coating of loosely attracted, electrically transferable material. As illustrated in Fig. l, the powder is deposited on the plate surface by a rotating powder-laden brush moving from left to right across the plate surface, the brush being triboelectrically negative with respect to the powder so as to deposit positively charged particles or powder on the plate surface.

In Fig. 2 an appropriate electric charge is placed on the deposited powder, for example, by passing a corona discharge electrode 19 across the plate surface. A suitable corona discharge electrode is disclosed in Walkup Patent 2,777,957 and may comprise one or more fine conductive strands 29 positioned within a backing shield 22 and maintained at a corona discharge potential by high voltage source 21 and optionally additionally shielded or screened by an array of conductors between the corona discharge wires and the surface being charged. The fine conductive wires are maintained at a desired corona discharge potential such as, for example, a potential in the order of several thousand volts and may be controlled to deposit on the surface electric charge of the desired polarity. As illustrated in Fig. 2, the corona discharge electrode is being moved across the plate surface from left to right to deposit positive polarity charge on the developer powder previously deposited on the plate surface. In this step or operation an electric charge of the desired polarity and of a sufficient potential of at least about 100 volts for presently known photoconductors and preferably 500 to 800 volts is placed on the layer of powder or developer material, in the event that said layer does not already possess electric charge of the desired amount and polarity.

In Fig. 3 is illustrated exposure of a plate prepared according to Figs. 1 and 2. In this operation a transparency 15, having opaque areas 15a and transparent areas 15b, is positioned between a light source 23 and plate 12. Light source 23 directs rays of light 18 to transparency l5 and through the transparent areas 15b to and through transparent support 14 and conductive layer 13 to photoconductive insulating layer 11. As a result of this exposure operation, the bound charges between photoconductive insulating layer 11 and conductive layer 13 are released in exposed areas and travel to the surface of the photoconductive insulating layer 11, as illustrated. Since in opaque areas 15a of transparency to" light rays 13 do not reach the photoconductive insulating layer 11, the insulating properties of this layer are maintained and the bound charges between photoconductive insulating layer 11 and conductive layer 13 remain bound in position.

In Fig. 4 is illustrated powder transfer and image formation on the surface of web 24. In this step the transparency used for exposure in Fig. 3 has been removed. The charge conditions, however, created during Fig. 3 continue. Thus, in plate 12 supported on backing support 14?- charge remains at the interface between conductor and photoconductive insulating layer 11 in areas which were not exposed. At the surfaces of photo conductive insulating layer 11 charge exists in areas which have been exposed. This charge has migrated to the surface from the interface between conductor 13 and photoconductor 11 because of the field conditions created by the particle layer on the surface of photoeonductive insulating layer 11 causing charge migration during exposure. Web 24 to which the powder is transferred during this step is rolled against the powder layer by conductive roller 24 connected to a potential source such as battery 26. With a proper potential applied as web 24 rolls into contact with the powder layer while web 24 is against roller 25, a point of separation during the coming together is reached where the intensity of the electric fields existing in relation to the powder on the surface of photoconductive insulating layer 11 in the areas which have not been exposed causes breakdown of the gap between web 24 and the powder layer as indicated in the drawing by the and signs and the arrows showing their direction of movement. The effect of breakdown at this point is to reverse the polarity of charge on the powder particles in the zone of breakdown. Thus, particles which are originally positive in polarity become charged negatively in areas not exposed creating in the powder layer image charge variations defined by the polarity of charge on the powder particles following passage through the zone of breakdown. Fortunately, the field for breakdown occurs at a point (believed to be in the range of from 90 to 200 microns) prior to attaining field conditions to bring about particle transfer. Thus, as illustrated in Fig. 4 following passage through the brakdown zone there results particle transfer of those particles whose charge polarity have not been affected during breakdown. The form of breakdown which takes place is known to change the charge polarity of the particles. and it is generallyv believed that as the web and backing roller electrode come-closer to the particle bearing plate a breakdown once started becomes quenched through field changes partially due to charges deposited because of breakdown and further, although the electric fields increase as the surfaces come closer together, the increase in fields is not great enough to sustain continued air breakdown for the narrower gaps involved. At a point beyond the breakdown zone electric field and gap conditions cause powder transfer of particles in the light exposed areas. Breakdown it is to be observed takes place preferentially in areas which have not been exposed. This occurs because of the greater electric fields which exist in these areas as compared to exposed areas. The effect of breakdown is realized during the web separation step when the particles whose charge polarity have been reversed are repelled from the transfer web while those continuing to hold their original charge are selectively transferred. As illustrated further in Fig. 4, web 24 may be separated from the surface of photoconductive insulating layer 11 at a subsequent time, or if desired it may be immediately separated following particle transfer against conductive roller 25. It is also to be realized that particle transfer and image formation may be carried out in accordance with the illustration of Fig. 4 by moving a biased conductively backed web while in substantially flat position toward the powder bearing plate 12 to cause the web to move into the first field zone of breakdown and the second field zone of field controlled particle transfer. Other modifications which readily occur to those skilled in the art are intended to be included herein and although the embodiment discussed in Figs. 1-4 is in terms of a particular polarity and field approach, it is to be realized that applying opposite polarity and opposite fields are intended to be within the scope of the instant invention. It is noted, however, that the particular polarity and field approach illustrated in Figs. 14 is preferred and that when operating in accordance with the steps illustrated in Figs. 1-4, it has been found a bias voltage of from 500 to 1500 volts produces reasonably good quality images with best images being produced applying a bias of l000 volts. When using a positive bias on roller 25 (the particles layer being charged negatively) voltages ranging from 1509 to 3000 volts positive have produced reverse images. None of the reversed images produced using a positive bias on electrode 25 were as good as the images produced when using negative biases.

Web 24 illustrated in this figure may be a cut sheet or continuous web and may be an electrical conductor such as a metallic sheet, foil, plate or the like in which case the web material itself may act as the field generating means thus dispensing with roller 25. It may also be an insulating web in which case it is preferred that the web material be quite thin. Alternatively, it may be a laminated plastic metal layer or a laminated paper metal layer and desirably print support member 24 comprises a sheet of paper conditioned in air at a relative humidity above about 20% and preferably above 40% so as to impart mild lateral conductivity to the paper. Generally it may be said that either the print support member 24 or a backing electrode such as roller 25 behind print support member 24 must have an electrical conductivity greater than that represented by a resistivity of 1 10 ohms-cm.

In Fig. 5 is illustrated an automatic machine for the production of photographic reversal copy according to the present invention. Generally illustrated within a light-tight cabinet 29 is a xerographic cylinder 30 comprising a conductively coated support cylinder such as, for example, a cylinder of Nesa glass, or the like. In an image area comprising a segment of the cylinder surface, the xerographic cylinder is coated with a photoconductive insulating layer 3 1 such as a layer of selenium or the like, and in the remainder of the surface area thecylinder is substantially transparent. The

.5: cylinder is suitably rotatably mounted and provided with drive means such as, for example, an electric motor or the like, or, optionally, manual drive mechanism to cause the cylinder to be rotated.

Positioned at an appropriate point around the circumference of the xerographic cylinder is an exposure station 32. There may be, for example, suitable lighting means such as fluorescent tubes mounted within the cylinder at the exposure station. Light shields 34 are positioned to protect the photosensitive portion of the drum from undesired exposure to the light of the exposure light source.

Mounted outside the cylinder 30 at the exposure station and at a point immediately after the line of exposure is a rol.er 36 adapted to be moved in and out of position in contact with the xerographic drum surface. Preferably, suitable cam 37, operating through rocker arm 38, moves the roller 36 into contact with the drum through 180 of rotation so as to bring the roller into contact only at those points which are not coated with the photoconductor.

Mounted within the cylinder is a suitable optical system adapted to focus a split projection image of the original onto the opposite surface of the cylinder at a position removed from the exposure zone by 180". This optical system may, for example, comprlse a roof mirror or prism 39, a reversing mirror 40, and a lens .1 so aimed and adjusted as to project and focus the image at the desired point. An original 42 to be copied may be fed between the cylinder and the drum and .'s carried through the exposure station by the rotating cylinder and roller in co-action. Suitable paper feed means 43 is operably positioned to feed the sheets of the original into roller 36 and cylinder 30.

Positioned adjacent the circumference at a point 180 from the exposure station is an image formation station generally designated 44. At this station is a suitable roller 4-5 having a soft, resilient surface such as, for example, a thick sponge rubber layer 46 which desirably is electrically conductive. Conductivity may be imparted to otherwise non-conductive rubber by impregnation with a conductive material such as, for example, a graphite dispersion. A suitable cam and rocker arm combination 48 is positioned and adapted to move roller 45 into and out of contact with the xerographic cylinder essentially simultaneously with roller 36. A suitable copy paper 47 is adapted to be fed between the sponge rubber roller 45 and the xerographic cylinder and to be carried through simultaneously with and in reverse direction from the original being copied. The copy paper may, as desired, be sheet fed or may as illustrated be web fed from feed roll 49 to take-up roll 50. As exposure frame or slit 51 is defined by shield edges 52 and 53, these edges being adjustable in position so as to control the width of the exposure slit opening. Thus, as the original is being fed against the surface of the xerographic cylinder at one point, an image thereof is being focused onto the opposite surface of the cylinder through the optical system and slit at a position 180 removed from the original. As illustrated in Fig. 5, roller 45 is positioned at a point just past exposure to allow for the change in charge conditions brought about by exposure as illustrated in Fig. 3 and to thus create charge conditions to bring about air breakdown and particle transfer in accordance with this invention.

Mounted between the exposure and image formation stations and shielded from both the exposure light source and the projected image is an operational charging station 54. At this charging station is positioned suitable charging means such as, for example, a corona discharge electrode 55 of the type described in conjunction with Fig. 2. This corona electrode is connected to a first polarity voltage source such as the positive terminal of a suitable high voltage power supply source, as shown in Fig. 2, and is adapted to deposit positve polarity electric charge on the surface of the photoconductor.

Optionally on the opposite side of the image formation station and subsequently thereto in the direction of rotation of the cylinder is an opposite charging electrode such as, for example, a corona discharge electrode 57 connected to the opposite or negative polarity terminal of a power supply as for example the power supply 21 of Fig. 2. Cooperatively mounted within the cylinder at the same point in the path of rotation, or at a subsequent point, is a flood light source such as, for example, a fluorescent tube 58 mounted within a light shield 59 being positioned and adapted to shine light through the Xerographic cylinder onto the photoconductive insulating layer.

Positioned next adjacent to this opposite charging electrode 57 is a developer supply means 61 comprising, for example, a rotatable brush 62 hearing a supply of finely divided developer material optionally maintained in supply by brushing against a developer-feed hopper 63 which is adapted to supply additional developer material thereto. The brush is movably mounted to be brought into and out of contact with the rotating cylinder with suitable cam and rocker arm 61 adapted to move the brush into contact only with the photoconductor coated portion of the cylinder.

Suitably mounted or positioned within the cabinet 29 is a source of direct current potential 67. One pole of this potential or voltage source is electrically connected to cylinder 30, as, for example, by means of a common electric ground and the other pole is electrically connected through a potentiometer 68 to roller 46, for example, by means of a connection to cam 48 or other suitable electrically conductive mounting member.

The operation of this and other apparatus to produce positive-to-positive photographic copy is disclosed in copending application Ser. No. 613,333.

In Fig. 6 is illustrated a simple diagrammatic DC voltage source suitable to supply operating voltages to the apparatus in either Figs. 1-4 or Fig. 5. The power supply includes a power transformer 70, a rectifier tube 71, and a filter circuit generally designated 72, operably connected across potentiometers 73, 74, 76 and '77 in series. Preferably potentiometer 74 is center-tapped to ground. The variable steps of potentiometers 73 and 74 are desirably fed to a selector switch 75 to supply a high voltage transfer position, a low voltage transfer position, and a floating position. The high positive and high negative potentiometers 76 and 77 are appropriately connected to operate the positive and negative corona electrodes 55 and 57 of Fig. 5.

Desirably, the voltage supply is capable of giving a DC. output in the order of about 6500 to 7500 volts to each corona electrode and a transfer potential of about 2000 volts. Potentiometers 73 and 74 desirably will have resistance ratios in the order of about 20 to 1 whereby the high transfer voltage tap can conveniently cover voltage ranges between about 500 volts and about 2000 volts and the low voltage tap can cover the voltage ranges from about +50 volts to about -50 volts. By feeding the selector switch voltage to electrode 23 of Fig. 4 or the transfer roller 46 of Fig. 5, the apparatus can produce in the high voltage position a photographic reversal print or in the floating position a direct photographic print. If voltage regulation for purposes such as background or density control is desired, this regulation may be achieved in the low voltage position. Thus, the apparatus illustrated in Fig. 5 can produce, at will, direct positive or photographic reversal prints by simple selector switch operation.

The present invention is beneficial to the art of xerography in that it results in simplification of the basic process manipulations. Thus, in carrying out image formation in accordance with the present invention there is eliminated the plate cleaning step generally required in the xerographic process. In accordance with the prescut invention the plate is continuously toned or main? tained in a loaded condition. There is no need for cleaning brushes, dust filters, driving motors or the like. Doing away with cleaning also improves apparatus according to this invention over the art by cutting down on vibration, reducing the amount of space consumed by the overall device and reducing also on plate wear and tear. The instant invention has the further advantage of simplifying developer systems. Rather than cascade development swab development or the like may be employed and it is to be realized that although the present invention has been described as carried out in specific embodiments, it is not desired to be limited thereto but it is intended to cover the invention broadly within the spirit and scope of the appended claims.

I claim:

1. The method of forming a Xerographic reversal print comprising depositing a layer of developer powder on the surface of a photoconductive insulating layer, uniformly charging said layer of developer powder to a first polarity, subsequently exposing the photoconductive insulating layer to an image pattern of light and shadow, and thereafter bringing a transfer web into contact with said powder layer while simultaneously applying an electric field in the same direction as that resulting from the charge on the powder layer between said transfer web and said powder layer, said field being sufiiciently intense to electrically break down the gap between said transfer web and said powder layer as said transfer web is brought to said powder layer in areas of said layer corresponding to shadow areas of said image pattern.

2. The method of forming a Xerogr'aphic reversal print comprising, in sequence, deposlting'a layer of developer powder on the surface of a photoconductive insulating layer, uniformly charging said layer of developer to a first 'polarity, exposing the photoconductive insulating layer to an image pattern of light and shadow, bringing a transfer weh into contact with said charged powder layer while simultaneously applying an electric field in the same direction as that resulting from the charge on the powder layer between said transfer web and said powder layer, said fields being sufiiciently intense to electrically break down the gap between said transfer web and said powder layer as said transfer web is brought to said powder layer in areas corresponding to shadow areas of said image pattern to recharge said areas of said powder layer to a second polarity opposite to said first polarity, and separating said transfer web from said powder layer carrying a particle image conforming in configuration to the light areas of said image pattern.

3. The method of forming a Xerographic reversal print comprising, in sequence, forming a uniformly positively electrostatically charged developer powder layer on the surface or" a photoconductive insulating layer, exposing the photoconductive insulating layer to an image pattern of light and shadow, and bringing a transfer web backed by a conductive electrode into contact with said powder layer while simultaneously applying a negative bias with respect to said photoconductive insulating layer to said backing electrode, said bias being sufficiently intense to bring about electrical breakdown of the gap between said transfer web and said powder layer as said transfer web is brought to said powder layer in areas corresponding to shadow areas of the image pattern which were not exposed during exposure of the photoconductive insulating layer to said image pattern.

4. The method of forming a Xerographic reversal print comprising, in sequence, forming a positively electrostatically charged developer powder layer, uniformly charged to at least about 100 volts, on the surface of a photoconductive insulating layer overlying a conductive backing electrode, exposing the photoconductive insulating layer to an image pattern of light and shadow, bringing a transfer web backed by a conductive electrode into contact with said powder layer while simultaneously applying a bias of from about -50() to about l500 volts to said web backing electrode with respect to said conductive backing electrode, said bias bringing about electrical breakdown of the gap between said transfer web and said powder layer as said transfer web is brought to said powder layer in areas of said powder layer corresponding to shadow areas of the image pattern to recharge said areas of said powder layer to a negative polarity, and separating said transfer web from said powder layer carrying a particle image on the surface of said transfer web conforming in configuration to the light areas of said image pattern.

5. The method of forming a xerographic reversal print comprising, in sequence, forming a uniformly positively electrostatically charged developer powder layer, charged between about 500 volts and 800 volts, on the surface of a photoconductive insulating layer overlying a conductive backing electrode, exposing the photoconductive insulating layer to an image pattern of light and shadow, br.nging a transfer web backed by a conductive electrode into contact with said powder layer while simultaneously applying a bias of about l,000 volts to said web backing electrode with respect to said conductive backing electrode, said bias bringing about electrical breakdown of the gap between said transfer web and said powder layer as said transfer web is brought to said powder layer in areas corresponding to shadow areas of the image pattern to recharge said areas of said powder layer to a negative polarity, separating said transfer web from said powder layer carrying a particle image on the surface of the transfer web conforming in configuration to exposed areas of said photoconductive insulating layer.

6. The method of forming a Xerographic reversal print comprisng, in sequence, forming a uniformly electrostatically charged layer of developer powder across the surface of a photoconductive insulating layer, exposing the photoconductive insulating layer to an image pattern of light and shadow, bringing a transfer web into contact with said powder layer while simultaneously applying an electric field in the same direction as that resulting from the charge on the powder layer between said transfer web and said powder layer, said field being sufiiciently intense to electrically break down the gap between said transfer web and said powder layer as said transfer web is brought to said powder layer in areas of said layer corresponding to shadow areas of said image pattern, and separating said transfer web from said powder layer carrying a particle image conforming in configuration to the light areas of said image pattern.

7. A method of forming a Xerographic reversal print comprising, in sequence, forming a uniformly electrostatically charged layer of developer powder across the surface of a photoconductive insulating layer, exposing the photoconductive insulating layer to an image pattern of light and shadow, bringing a transfer web into contact with said powder layer while simultaneously applying an electric potential of more than about 500 volts in the same direction as that resulting from the charge on the powder layer between said transfer web and said powder layer, and separating said transfer web from said powder layer carrying a particle image conforming in configuration to the light areas of said image pattern.

2,758,525 Montcriefi-Yeates Aug. 14, 1956 

1. THE METHOD OF FORMING A XEROGRAPHIC REVERSAL PRINT COMPRISING DEPOSITING A LAYER OF DEVELOPER POWDER ON THE SURFACE OF A PHOTOCONDUCTIVE INSULATING LAYER, UNIFORMLY CHARGING SAID LAYER OF DEVELOPER POWDER TO A FIRST POLARITY, SUBSEQUENTLY EXPOSING THE PHOTOCONDUCTIVE INSULATING LAYER TO AN IMAGE PATTERN OF LIGHT AND SHADOW, AND THEREAFTER BRINGING A TRANSFER WEB INTO CONTACT WITH SAID POWDER LAYER WHILE SIMULTANEOUSLY APPLYING AN ELECTRIC FIELD IN THE SAME DIRECTION AS THAT RESULTING FROM THE CHARGE ON THE POWDER LAYER BETWEEN SAID TRANSFER WEB AND SAID POWDER LAYER, SAID FIELD BEING SUFFICIENTLY INTENSE TO ELECTRICALLY BREAK DOWN THE GAP BETWEEN SAID TRANSFER WEB AND SAID POWDER LAYER AS SAID TRANSFER WEB IS BROUGHT TO SAID POWDER LAYER IN AREAS OF SAID LAYER CORRESPONDING TO SHADOW AREAS OF SAID IMAGE PATTERN. 